1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer, direct digital master making machine or similar electrophotographic image forming apparatus. More particularly, the present invention relates to a developing device of the type developing a latent image with a magnet brush and an image forming apparatus using the same.
2. Description of the Background Art
Generally, an electrophotographic image forming apparatus includes an image carrier implemented as a photoconductive drum or a photoconductive belt. A developing device develops a latent image formed on the image carrier to thereby produce a corresponding toner image. It is a common practice with this type of image forming apparatus to use either one of a one-ingredient type developer or toner and a two-ingredient type developer, i.e., a mixture of toner and carrier grains. Development using the two-ingredient type developer features desirable image transferability and desirable developing characteristics against temperature and humidity. The two-ingredient type developer forms brush chains on a developer carrier in a developing zone where the developer carrier faces the image carrier. The toner is fed from the developer on the developer carrier to a latent image formed on the image carrier.
As for the development using the two-ingredient type developer, a decrease in the distance between the image carrier and the developer carrier in the developing zone allows high image density to be easily attained and reduces so-called edge effect. This, however, is apt to cause the trailing edge of a black solid image or that of a halftone solid image to be lost. Let this undesirable phenomenon be referred to as the omission of a trailing edge hereinafter.
The omission of a trailing edge can be reduced if a nip where the magnet brush contacts the surface of the image carrier is reduced in width in the direction of movement of the above surface, as reported in the past. The omission of a tailing edge can be further reduced if the magnet brush is dense at the nip, as also reported in the past.
Further, when the magnet brush is cut off on contacting the image carrier due to a shock, an electric force greater than a magnetic force prevents the magnetic carrier to be returned to the developer carrier. As a result, the magnetic carrier deposits on the non-image portion of the image carrier. Assume that image transfer and fixation are effected after development with the carrier depositing on the image carrier. Then, the carrier is transferred from the image carrier to a sheet together with a toner image and therefore scratches a fixing roller, resulting in a defective image.
On the other hand, the developer is agitated for uniformly distributing the carrier and toner. However, stress ascribable to agitation increases the stress of the toner and carrier, increasing the amount of charge. It is therefore necessary to raise a bias for development. Such a bias, however, reduces the service life of the image carrier.
If the magnetic force of the developer carrier is increased to surely retain the carrier, then heat is generated due to the rotation of the developer carrier or the torque for driving the developer carrier must be increased. In addition, it is difficult to reduce the size of the developer carrier.
The amount of charge deposited on the toner and carrier noticeably varies at the time of toner transfer. Consequently, the carrier is apt to scatter until the restoration of the original amount of charge.
Moreover, when the linear velocity of the developer carrier is increased, the agitation of the toner becomes short and causes the toner with a non-uniform distribution to reach the developing zone between the image carrier and the developer carrier. Particularly, in the case of a halftone image, the non-uniform toner distribution is apt to bring about irregular image density in the vertical direction of an image. It is known that irregular image density in the vertical direction is ascribable to minute differences in the toner content of the developer in the lengthwise direction.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-197833, 10-48958, 2000-231258, 2000-305360, and 2001-5296.
It is a first object of the present invention to provide a developing device capable of uniformly distributing toner on a developer carrier, and an image forming apparatus using the same.
It is a second object of the present invention to provide a developing device capable of obviating carrier deposition, and an image forming apparatus using the same.
A developing device of the present invention includes a developer carrier for conveying a developer, which consists of toner and magnetic carrier, deposited thereon. In a developing zone, the developer carrier causes the developer to form a magnet brush with a main magnetic line of force issuing from a main magnetic pole, which is positioned inside the developer carrier. The toner is fed to an image carrier in the developing zone. A first metering member is positioned upstream of the developing zone in the direction of developer conveyance for regulating the amount of the developer being conveyed by the developer carrier toward the developing zone. A space is formed for collecting part of the developer removed by the first metering member. A toner hopper adjoins the above space for replenishing fresh toner to the developer carrier. A second metering member is positioned upstream of the first metering member in the direction of developer conveyance. A gap is formed between the second metering member and the developer carrier for preventing, when the toner content of the developer on the developer carrier increases to increase the thickness of the developer forming a layer on the developer carrier, the increment of the developer carrier from passing. A condition in which the developer and fresh toner contact each other varies in accordance with the toner content of the developer present on the developer carrier, varying the condition of replenishment of the fresh toner to the developer. At least one auxiliary magnetic line of force is positioned between the developing zone and the first metering member.
An image forming apparatus including the above developing device is also disclosed.